MDEV-21028 Server crashes in Query_arena::set_query_arena upon SELECT from view

if the view has algorithm=temptable it is not updatable,
so DEFAULT() for its fields is meaningless,
and thus it's NULL or 0/'' for NOT NULL columns.

mysql-test/r/func_default.result

@@ -159,5 +159,48 @@ a
 10
 DROP TABLE t1;
 #
+# MDEV-21028 Server crashes in Query_arena::set_query_arena upon SELECT from view
+#
+create table t1 (a datetime default current_timestamp);
+insert into t1 () values (),();
+create algorithm=temptable view v1 as select * from t1;
+create algorithm=merge     view v2 as select * from t1;
+select default(a) = now() from v1;
+default(a) = now()
+NULL
+NULL
+select default(a) = now() from v2;
+default(a) = now()
+1
+1
+select table_name,is_updatable from information_schema.views;
+table_name	is_updatable
+v1	NO
+v2	YES
+drop view v1, v2;
+drop table t1;
+create table t1 (v1 timestamp) select 'x';
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `v1` timestamp NOT NULL DEFAULT current_timestamp() ON UPDATE current_timestamp(),
+  `x` varchar(1) NOT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+select default(v1) from (select v1 from t1) dt;
+default(v1)
+2001-01-01 10:20:30
+select default(v1) from (select v1 from t1 group by v1) dt;
+default(v1)
+0000-00-00 00:00:00
+drop table t1;
+create table t1 (a text default '');
+create algorithm=temptable view v1 as select * from t1;
+insert into t1 values ('a');
+select default(a) from v1;
+default(a)
+NULL
+drop view v1;
+drop table t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/func_default.test

@@ -139,6 +139,32 @@ FROM t1;
 SELECT a FROM t1 WHERE CASE WHEN a THEN DEFAULT(a) END IS FALSE;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-21028 Server crashes in Query_arena::set_query_arena upon SELECT from view
+--echo #
+create table t1 (a datetime default current_timestamp);
+insert into t1 () values (),();
+create algorithm=temptable view v1 as select * from t1;
+create algorithm=merge     view v2 as select * from t1;
+select default(a) = now() from v1;
+select default(a) = now() from v2;
+select table_name,is_updatable from information_schema.views;
+drop view v1, v2;
+drop table t1;
+
+create table t1 (v1 timestamp) select 'x';
+show create table t1;
+select default(v1) from (select v1 from t1) dt;
+select default(v1) from (select v1 from t1 group by v1) dt;
+drop table t1;
+
+create table t1 (a text default '');
+create algorithm=temptable view v1 as select * from t1;
+insert into t1 values ('a');
+select default(a) from v1;
+drop view v1;
+drop table t1;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

sql/sql_select.cc

@@ -16333,7 +16333,8 @@ Field *create_tmp_field_from_field(THD *thd, Field *org_field,
       table->s->db_create_options|= HA_OPTION_PACK_RECORD;
     else if (org_field->type() == FIELD_TYPE_DOUBLE)
       ((Field_double *) new_field)->not_fixed= TRUE;
-    new_field->vcol_info= 0;
+    new_field->vcol_info= new_field->default_value=
+      new_field->check_constraint= 0;
     new_field->cond_selectivity= 1.0;
     new_field->next_equal_field= NULL;
     new_field->option_list= NULL;